It is known to preserve manuscripts by application of a polymer coating. Currently practiced methods of coating a paper surface with such a film involve at least seven distinct stages:
synthesis of a monomer; PA1 polymerization of the monomer with formation of intermediate or end polymer; PA1 preparation of a film forming solution; PA1 cleaning of the surface or application of a bonding agent to the surface; PA1 application of the coating; PA1 drying of the coating; PA1 solidification of the coating. PA1 paper tends to become somewhat more shiny in appearance; PA1 paper treated with parylene tends to feel slippery, because of the dry film lubricity of the material. In this respect, it is similar to Teflon; PA1 documents treated with parylene have areas with faint rainbow-colored patterns which indicate localized areas of film thinning caused by extreme challenges to gas penetration; PA1 papers with heavy applications of parylene (12 .mu.m or more) sometimes develop a rough texture or feel; PA1 some color shade changes can occur in parylene-treated papers. PA1 1. plasma treatment in a glow discharge in an atmosphere of H.sub.2, CO, N.sub.2, O.sub.2 gases; PA1 2. plasma polymerization; and PA1 3. treatment in plasma of hydrogen. PA1 generator frequency 1 to 40 MHz PA1 pressure 0.01 to 10 Torr PA1 specific discharge power 0.003 to 3 wt/cm.sup.3 PA1 treatment times--30 to 3600 sec.
The basic disadvantages of these methods include the large number of stages involved in the process as well as unevenness and excessive thickness of the resultant coating, which leads to a change in the appearance of the object being preserved.
Another known approach to the archival preservation of documents and archaeological materials is the Parylene treatment method developed by Union Carbide. Parylene has come to be known as the generic term for a family of polymers derived from common xylene--the polyparaxylylenes. They are the only polymer group which forms in a vacuum from a true gas phase. Parylene comes to the end user in the dimeric form, as a free-flowing powder. This material must be converted to the final polymeric form within a special vacuum deposition system. Stages required to apply a coating to a paper surface using the parylene method include:
The dimer, when placed in a vacuum and heated to about 120 deg. C., begins to sublime, forming dimeric parylene gas.
In a pyrolysis zone the dimeric gas molecule is split into two reactive monomer molecules by the 650-690 deg. C. temperature.
Pressure forces the monomer gas through a pyrolysis zone and out into a deposition chamber (at room temperature).
The monomer molecules pick up very high kinetic energy during their passage through the heated zones. As a result, they bounce around the chamber hundreds to thousand of times before losing enough energy to absorb and polymerize on a surface within the chamber. This growth process results in long chains (linear polymer) that do not cross-link.
This method is characterized by polymerization beneath as well as on the surface of the growing film. The polymerization process occurs at essentially ambient temperature and there is no liquid phase, solvents or plasticizers. (Paper Strengthening with Gas-Phase Parylene Polymers: Practical Considerations.; Humphrey B., Restaurator 11: 1990, Munksgaard, Copenhagen).
The structure of the molecule and the high kinetic energies imparted during the process result in deep penetration into porous substances. The gas phase nature of the process as well as the growth from a molecular scale give unparalleled conformity of coating even on very complex substrates.
These characteristics make this material suitable for conservation applications ( The application of parylene conformal coating technology to archival and artifact conservation; Humphrey, B., Studies in Conservation 22 (2): August, 1984).
However, the process is generally not reversible on most substrates, particularly paper. The coating material is not soluble and forms a tight bond to most substrates, making removal difficult, if not impossible (Humphrey B: Y. Am. Ass. Cons. Hist. Art. Wks, 1986, v. 25 (2) pp. 15-22). The net result of the process is a new material that is no longer purely paper. What is produced is a parylene-cellulose composite with entirely different physical properties. The new material still has the same general appearance of paper but now, in addition, has the properties of parylene as well.
The parylene-cellulose composite is extremely resistant to chemical attack by all organic and inorganic chemicals at ambient temperature. The paper is extremely hydrophobic and can withstand total immersion in water for years with no damage to print or paper. The parylene-cellulose composite has reduced permeability to water vapor and harmful gases, i.e. H.sub.2 S, SO.sub.2, and Cl.
Parylene changes the appearance of the paper thus destroying the historical value of the document being preserved. Specifically,
Once treated, Parylene technology does not allow further restoration of the archive documents. An additional disadvantage of the method is that it involves multiple stages.
Plasma polymerization techniques have been used in the past for certain applications unrelated to the objectives of this invention.
Japanese patent 63-75002 described treatment in an impulse or pulsed discharge in an atmosphere comprising the gases CH.sub.4, C.sub.2 H.sub.6 or C.sub.4 H.sub.10 for increasing the durability and thermal stability of ferromagnetic layers of magnetic tapes. This method cannot be applied to the preservation of manuscripts and the like because the film formed during the process changes the appearance of the treated surface.
Another prior method of achieving film plasma polymerization, described in U.S. Pat. No. 4,188,426, includes treatment in a glow discharge of per-fluoro-cyclo-butane or hexafluoroethane to reduce the friction coefficient and to improve the surface hydrophobia of organic and inorganic substrates (e.g. polyethylene films, metals). This method also cannot be applied to conservation of manuscripts because the film formed during the process changes the appearance of the treated surface. In addition, the use of fluoro-containing monomers is contraindicated by ecological considerations.
A known method of water and oil repellent finishing of textiles, described in USSR Patent 1,158,634, includes plasma treatment in a glow discharge in an atmosphere of inorganic gases, followed by treatment with a fluoro containing acrylic monomer in gas phase . The first stage of the process can cause additional destruction of archival documents when the documents interact with the gas that creates the plasma. The second stage forms too rough a film.
Another prior method of plasma formation of a thin film on the surface of polymer material, described in Japanese Patent 62-132940, includes:
The first stage is used to improve adhesion of the film surface for the subsequent polymerization stage. This first stage lasts from 20 sec to 30 minutes of time and can cause additional destruction of archive documents when the documents interact with the gas that creates the plasma.
What is needed is a conservation method for use on materials such as paper manuscripts which do not alter the appearance nor physically damage the item being preserved, which involves a minimum of processing of the item, which can be safely used on various materials, which is not ecologically damaging, and which is simple and reliable.